1. Field of the Invention
The present invention relates to a module including an external connection conductor for used in external connection and a method of manufacturing the module.
2. Description of the Related Art
There is a known module that includes an external connection conductor for use in external connection, as illustrated in FIG. 7 (see, for example, paragraph Nos. 0005 to 0008 and 0012 and FIG. 16 in Japanese Unexamined Patent Application Publication No. 2001-94033). The module 100 includes a circuit board 101 with wiring electrodes formed inside and on its surface, components 102 mounted on a first principal surface of the circuit board 101, external connection conductors 103 for use in external connection, and a resin layer 104 disposed near the first principal surface of the circuit board 101 and covering the components 102 and the external connection conductors 103. The external connection conductors 103 have first ends connected to the circuit board 101 and stand on the first principal surface of the circuit board 101. The external connection conductors 103 have second end faces exposed through the surface of the resin layer 104, and solder balls 105 are formed on their respective second end faces. The external connection conductors 103 and an external mother board or other element are connected to each other through the solder balls 105, and the module 100 and the mother board are electrically connected to each other.
In the above-described known module 100, the second end face of each of the external connection conductors 103 and the surface of the resin layer 104 lie in the same plane, that is, the second end face of the external connection conductor 103 and the surface of the resin layer 104 are flush with each other, as illustrated in FIG. 7. Thus, for example, if an external stress or an internal stress, such as the one caused by heat, is exerted on the module 100 in a state where the module 100 is connected to an external mother board, the stress concentrates on the interface between the external connection conductor 103 and the solder ball 105. That is, the stress-concentrated portion and the interface overlap with each other.
The external connection conductor 103 and the solder ball 105 are considered to be connected to each other with an intermetallic compound formed by interdiffusion of the conductive material of the external connection conductor 103 and the solder in the above-described interface. This intermetallic compound typically has a low mechanical strength and is fragile. Thus if an external stress or an internal stress, such as the one caused by heat, is exerted on the module 100 in a state where the module 100 is connected to a mother board or other element, the connection between the external connection conductor 103 and the solder ball 105 may be cracked or ruptured from the interface between them as a starting point. This leads to a problem in connection reliability.